Sometimes it's hard to appreciate your siblings. As children, you fought over toys and car seats; as adults, you argue over perceived slights, or shared responsibilities. But April 10 is National Sibling Day, which means it's time to put away hard feelings and remember why brothers and sisters are so special. Here are 11 quotes and scientific facts about the power of siblings and sibling relationships.

1. Siblings spend a lot of time together: By the age of 11, children devote one-third of their free time to their brothers and sisters, a 1996 Penn State University cited in TIME showed. That amount of time is even more than they give to their friends, teachers and parents -- and themselves.

2. Siblings teach us social skills: Fighting with your brother or sister as a child was actually a good thing. According to at least one study on kindergartners, those with siblings got along better with classmates than those without. 

3. Birth order matters: Firstborns are supposedly the most successful and the most driven to succeed, according to Jeffrey Kluger, author of The Sibling Effect.

4. The sex of your siblings matters: Boys who grow up with sisters have better listening skills and are more sensitive, Kluger said in an interview with Salon, while girls that grew up with brothers were less serious. Having a sibling of the opposite sex can also help you in romantic relationships with people of the opposite sex.

5. Relationships with our siblings are often the longest ones of our lives: For the most part, relationships with siblings often start and end with life itself, outlasting even our relationships with our parents, as the Siblings Day Foundation points out.

And now for the quotes, to share with your siblings to show how much you appreciate them:

6. "I don’t believe an accident of birth makes people sisters or brothers. It makes them siblings, gives them mutuality of parentage. Sisterhood and brotherhood is a condition people have to work at." -- Maya Angelou

7. “You can kid the world, but not your sister.” -- Charlotte Gray

8. "Having a sister is like having a best friend you can't get rid of. You know whatever you do, they'll still be there." -- Amy Li

9. "It snowed last year too: I made a snowman and my brother knocked it down and I knocked my brother down and then we had tea." -- Dylan Thomas

10. "It takes two men to make one brother." -- Israel Zangwill

11. "Sibling relationships -- and 80 percent of Americans have at least one -- outlast marriages, survive the death of parents, resurface after quarrels that would sink any friendship. They flourish in a thousand incarnations of closeness and distance, warmth, loyalty and distrust." -- Erica E. Goode